Yume Nikki (Dream Diary) is a Japanese freeware game that has caught the attention of /jp/ for its surreal atmosphere and nightmare fuel. Gameplay is simple as it merely involves exploring 12 open ended dream worlds with no specific goal in mind but to collect "effects" which changes the character's appearance.
A disembodied head known as Uboa who transports the player to an endless stretch of map. The appearance of Uboa only has a small probability of being triggered, so even trying everything in the game once may not be enough to find Uboa. Youtube playthroughs showing how to get Uboa was one of the main reasons that sparked interest with /jp/ in the first place.